According to Den of Geek, the role of The Collector has gone to Del Toro. The character has been around since 1966 so naturally he has a long, rich, and confusing biography. As for how he might factor into the film based on his relationship to Thanos in the comics, Wikipedia provides this bit from The Collector’s history:

The Collector also had the power of prophecy, allowing him to foresee the rise of a being powerful enough to pose a threat to the Elders: Thanos. To protect life in the universe, the Collector created a massive museum of countless life forms to keep them safe from Thanos. For a time, he even possessed one of the six Infinity Gems, unaware of its true power, until Thanos took it.

While this sounds somewhat benevolent, the Collector was an enemy of The Avengers in the modern age, and had to be defeated on more than one occasion.

We also previously reported that the Collector would be “villainous”, and that Pace would play his aide. Den of Geek reports that Lee is Ronan the Accuser, a member of the alien Kree race and son of Thanos in Marvel’s “Ultimate” universe.

Also in Thanos’ family tree, Den of Geek says that Gillan is Nebula, an evil space pirate and mercenary who claimed that Thanos was her grandfather. Looking over her character bio on Wikipedia, her relationship to Thanos in the comics is closer than any other character.

But that’s the comics, and that’s the most important thing to remember. As we saw earlier this year with the Mandarin reveal, the comics are not holy writ. Assuming these roles are correct, we don’t know how they’ll relate to each other. Guardians of the Galaxy is bordering on unwieldy when you consider the amount of characters and mainstream audiences’ unfamiliarity with all of them. Perhaps Marvel will straighten some things out at their Comic-Con panel, but I seriously doubt they’ll give us a particularly clear idea of exactly what director James Gunn is up to with his massive cast.